The ability to isolate viral DNA and clone this isolated DNA into bacterial plasmids has greatly expanded the approaches available to make viral vaccines. The methods used to make the present invention involve modifying cloned viral DNA sequences by insertions, deletions and single or multiple base changes. The modified DNA is then reinserted into the viral genome to render the virus non-pathogenic. The resulting live virus may then be used in a vaccine to elicit an immune response in a host animal and to protect the animal against a disease.
One group of animal viruses, the herpesviruses or Herpetoviridae, is an example of a class of viruses amenable to this approach. These viruses contain 100,000 to 200,000 base pairs of DNA as their genetic material. Importantly, several regions of the genome have been identified that are non-essential for the replication of virus in vitro in cell culture. Modifications in these regions of the DNA may lower the pathogenicity of the virus, i.e., attenuate the virus. For example, inactivation of the thymidine kinase gene renders human herpes simplex virus non-pathogenic (28), and pseudorabies virus of swine non-pathogenic (29).
Removal of part of the repeat region renders human herpes simplex virus non-pathogenic (30,31). A repeat region has been identified in Marek's disease virus that is associated with viral oncogenicity (32). A region in herpesvirus saimiri has similarly been correlated with oncogenicity (33). Removal of part of the repeat region renders pseudorabies virus non-pathogenic (U.S. Pat. No. 4,877,737, issued Oct. 31, 1989). A region in pseudorabies virus has been shown to be deleted in naturally-occurring vaccine strains (11,3) and it has been shown that these deletions are at least partly responsible for the lack of pathogenicity of these strains.
It is generally agreed that herpesviruses contain non-essential regions of DNA in various parts of the genome, and that modifications of these regions can attenuate the virus, leading to a non-pathogenic strain from which a vaccine may be derived. The degree of attenuation of the virus is important to the utility of the virus as a vaccine. Deletions which cause too much attenuation of the virus will result in a vaccine that fails to elicit an adequate immune response. Although several examples of attenuating deletions are known, the appropriate combination of deletions is not readily apparent.
Infectious bovine rhinotracheitis (IBR) virus, an alpha-herpesvirus with a class D genome, is an important pathogen of cattle. It has been associated with respiratory, ocular, reproductive, central nervous system, enteric, neonatal, and dermal diseases (34). Cattle are the normal hosts of IBR virus, however it also infects goats, swine, water buffalo, wildebeest, mink, and ferrets. Experimental infections have been established in muledeer, goats, swine, ferrets, and rabbits (35).
Conventional modified live virus vaccines have been widely used to control diseases caused by IBR virus. However, these vaccine viruses may revert to virulence. More recently, killed virus IBR vaccines have been used, but their efficacy appears to be marginal.
IBR virus has been analyzed at the molecular level as reviewed in Ludwig (36). A restriction map of the genome is available in this reference, which will aid in the genetic engineering of IBR according to the methods provided by the present invention.
As reported in the current literature, IBR virus has been engineered to contain a thymidine kinase deletion (43,44) and a deletion in the gIII gene (45,46). However, no evidence has been presented for the deletions in the US2, repeat, gpG, or gpE regions. In the subject application, we demonstrate the usefulness of such deletions for both the attenuation of IBR virus and for the development of gene deleted marker vaccines.
As with other herpesviruses, IBR virus can become latent in healthy animals which makes them potential carriers of the virus. For this reason it is clearly advantageous to be able to distinguish animals vaccinated with non-virulent virus from animals infected with disease-causing wild type virus. The development of differential vaccines and companion diagnostic tests has proven valuable in the management of pseudorabies disease (47). A similar differential marker vaccine would be of great value in the management of IBR disease. The construction of differential diagnostics has focused on the deletion of glycoproteins. Theoretically, the glycoprotein chosen to be the diagnostic marker should have the following characteristics: (1) the glycoprotein and its gene should be non-essential for the production of infectious virus in tissue culture; (2) the glycoprotein should elicit a major serological response in the animal; and (3) the glycoprotein should not be one that makes a significant contribution to the protective immunity. Four major IBR virus glycoproteins (gI, gII, gIII, and gIV) have been described in the literature (48). Three of these genes, gI, gIII, and gIV, have been sequenced and shown to be homologous to the HSV glycoproteins gB, gC, and gD, respectively. Although it has been suggested that the gII protein is analogous to HSV gE, no sequence evidence has been presented to confirm that suggestion (48). The gB and gD homologues are essential genes and would not be appropriate as deletion marker genes. The gC gene of herpesviruses has been shown to make a significant contribution to protective immunity as a target of neutralizing antibody (49) and as a target of cell-mediated immunity (50). Therefore, the gC gene is not desirable as a deletion marker gene. As indicated above, Kit et al. (45) have described the deletion of the IBR virus gIII as a marker gene. It would be expected that such a deletion would compromise the efficacy of an IBR vaccine.
For pseudorabies virus (PRV) the criteria for a deletion marker gene are best met by the glycoprotein X (51). Wirth et al. (52) suggests the existence of a "gX homologue of HSV-1" in the IBR virus. It is not clear what is meant by this because although there is a PRV gX gene, there is no reported HSV-1 gX gene or gX homologous gene. In any case, no sequence evidence is presented to support this suggestion. We present clear evidence of homologues of PRV gX (HSV-2 gG) and PRV gI (HSV gE) in IBR virus and demonstrate their usefulness as diagnostic markers.
The present invention provides a method of producing a fetal-safe, live recombinant IBR virus which comprises treating viral DNA from naturally-occurring live IBR virus so as to delete from the virus DNA corresponding to the US2 region of the naturally-occurring IBR virus. The present invention also provides viruses in which (1) DNA corresponding to the US2 region of naturally-occurring IBR virus has been deleted, and (2) DNA encoding gpG and/or gpE has been altered or deleted. Such viruses are useful in vaccines which need diagnostic markers and are safe for use in pregnant animals.
The ability to engineer DNA viruses with large genomes, such as vaccinia virus and the herpesvirues, has led to the finding that these recombinant viruses can be used as vectors to deliver immunogens to animals (53). The herpesviruses are attractive candidates for development as vectors because their host range is primarily limited to a single target species (54), and they have the capacity for establishing a latent infection (55) that could provide for stable in vivo expression of a desired cloned polypeptide. Herpesviruses have been engineered to express a variety of foreign gene products, such as bovine growth hormone (56), human tissue plasminogen activator (57), and E. coli .beta.-galactosidase (58,59). In addition, possible immunogenic polypeptides have been expressed by herpesviruses. Whealy et al. (60) expressed portions of the human immunodeficiency virus type 1 envelope glycoprotein in pseudorabies virus (PRV) as fusions to the PRV glycoprotein III. The hepatitis B virus surface antigen (61) and a hybrid human malaria antigen from Plasmodium falciparum have been expressed in herpes simplex virus type 1 (HSV-1) (62). The IBR viruses described above may be used as vectors for the insertion of genes encoding antigens from microorganisms causing important cattle diseases. Such recombinant viruses would be multivalent vaccines protecting against IBR as well as other diseases. Kit et al. (63) have described the expression of a Foot and Mouth disease antigen in IBR virus. In some of the prior applications from which the subject application claims priority (which precedes the Kit publication by at least three years), we described the use of IBR virus to express several foreign genes including the E. coli .beta.-galactosidase (lacZ) gene, the TN5 neomycin resistance gene, and antigens from bovine rota virus, and parainfluenza-3 virus (see U.S. Ser. No. 06/933,107, filed Nov. 20, 1986 and U.S. Ser. No. 07/078,519, filed Jul. 27, 1987). These applications precede the Kit publication by at least three years. The viruses described in this application provide a combination of attenuation, differentiation and multivalency. These properties make such viruses useful as vaccines for the management of cattle diseases.